Issue Description

The AI Artifacts Common contribution guide needs to be updated to comprehensively document the plugin and collection system that is currently in production. While the system is functional, the documentation lacks complete guidance on validation workflows, plugin generation, and the relationship between collections, extension packaging, and plugin outputs.

Contributors currently lack clear documentation on:

• Plugin validation and generation workflow
• Collection manifest validation using npm run plugin:validate
• How collections flow through extension packaging with agent handoff resolution
• Plugin directory auto-generation and the warning against direct edits
• Repo-specific exclusion rules for .github/**/hve-core/ artifacts

Acceptance Criteria

• Collections section will be restructured for improved learning flow (collections → extension packaging → plugin generation)
• Plugin validation workflow will be documented with npm run plugin:validate and npm run plugin:generate commands
• Validation checks will be enumerated (YAML syntax, required fields, path references, kind/maturity values, duplicates, hve-core exclusions)
• Plugin generation workflow will include clear steps for contributors adding artifacts to collections
• Critical warnings will be added about plugin directory auto-generation (do not edit plugins/ directly)
• Extension packaging section will explain agent handoff dependency resolution during build
• Collection manifest field documentation will be accurate (path, kind, maturity only - no deprecated fields)
• Repo-specific exclusion guidance will be expanded with validation enforcement details
• All documentation updates will pass npm run lint:md and npm run lint:frontmatter

Implementation Tasks

1. Restructure Collections section for logical flow (basics → extension → plugins)
2. Add Plugin Generation section with comprehensive workflow documentation
3. Document validation workflow with checks performed by npm run plugin:validate
4. Add Extension Packaging section explaining handoff resolution
5. Expand repo-specific exclusion guidance with validation enforcement
6. Add critical warning boxes about plugin directory auto-generation
7. Remove references to deprecated or non-existent fields (such as requires in collection manifests)
8. Verify documentation accuracy against actual scripts and collection files

Reference Documentation

Implementation references:

• scripts/plugins/Generate-Plugins.ps1 - Plugin generation implementation
• scripts/plugins/Validate-Collections.ps1 - Collection validation checks
• scripts/extension/Prepare-Extension.ps1 - Extension packaging and handoff resolution
• collections/*.collection.yml - Actual collection manifest schema in use

Additional Context

No response